When Castle (Nathan Fillion) and Beckett (Stana Katic) discover a murder victim who was apparently murdered by a rogue CIA agent, they quickly find themselves in the middle of a much larger investigation, led by Beals' Sophia Conrad."She is a CIA agent and, as we know, in Richard Castle's first series of books, a female CIA agent figured prominently," creator and executive producer Andrew W. Marlowe tells TVGuide.com. "So, of course, it would lead us to ask: Is Beckett Castle's first muse?"Naturally, Beckett learning that she isn't Castle's first will leave Beckett with lots of questions about the will-they-won't-they couple's future. "Beckett has to wrestle with where she thought she stood in Castle's life and where she might really stand in Castle's life," Marlowe says. "I think it's natural in a situation like that to feel threatened to some extent, and to measure yourself up."

"What [Sophia] meant to Castle at the time ... and whether or not it makes Beckett feel any less important in Castle's life, are all things we're dealing with," he continues. "Those are questions that Beckett is trying to have answered without exposing her feelings too much."For Katic's part, she believes her character will also be a little disturbed by Castle's repeat behavior. "When you find out that the guy has had other muses before you, it's not like, 'Oh, geez, am I muse enough?'" Katic says. "It's more like, 'Some guys have foot fetishes — is this your thing? You just follow women around and tell them they're your muse so that you can get in their pants?' I think that's the confusion there. "

So, could this bombshell set off other explosions in Castle and Beckett's relationship? After all, both of them are already keeping big secrets from one another."I think it serves as a bit of a touchstone for them," Marlowe says. "It certainly brings up stuff in both of our characters about how they're handling the relationship and getting a sense of what they want, what they're afraid of, and what they hope might happen."Castle airs Mondays at 10/9c on ABC. The two-parter begins on Feb. 13.